Data transfer control of a video memory having a multi-divisional random access memory and a multi-divisional serial access memory

ABSTRACT

An image memory has a random access memory array capable of being randomly accessed; a serial access memory array partitioned into 2 n  (n&gt;1) divisional areas cyclically and serially accessed asynchronously with the random access memory array; a data transfer unit to transfer data between the random access memory array and the serial access memory array; a determining unit to determine a row of data to be transferred from the random access memory array to each of the divisional areas; and a designating unit for designate at least one of a top serial access address and a last serial access address, respectively, of each divisional area. The data transfer unit executes data transfer from the random access memory array to the serial access memory array in accordance with outputs from the determining unit and the designating unit.

This application is a continuation of application Ser. No. 07/669,095, filed Mar. 13, 1991, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an image memory.

As memories suitable for high speed data processing in engineering work stations (EWSs) or computer graphics (CG), etc. and display associated therewith, attention in recent years is being drawn to dual port video random access memories (hereinafter referred to as DPRAMs). Such DPRAMs are provided with a random access port "RAM" standing for random access memory (hereinafter referred to as a RAM port) including a memory array accessible at random (e.g., a dynamic random access memory (DRAM), and a serial access port (hereinafter referred to as a SAM port "SAM" standing for serial access memory) including a serial access memory cyclically and serially accessed asynchronously with the RAM port. In such DPRAMs, since transfer of data is made between the RAM port and the SAM port, it is necessary that the timing at the RAM port and the timing at the SAM port be made synchronous with each other only at the time of this transfer cycle. The timings in this transfer cycle will be described with reference to FIGS. 6A and 6B. In the case of data transfer of a certain row R of a memory array 1 of the RAM port to a serial access memory 2 of the SAM port which is incessantly making a serial access (see FIG. 6A), a procedure is taken to produce a first external signal DT for controlling transfer (see time T₁ of FIG. 6B) fall. If the external signal DT is at "L" level at the time when the signal RAS falls, the operational mode enters the transfer cycle.

In this transfer cycle, in the same manner as that in an ordinary RAM cycle, a row address and a column address are given synchronously with falls of RAS and CAS (see times T₂ and T₃ of FIG. 6B), respectively. Unlike the ordinary RAM cycle, the row address indicates a row of the memory array 1 to be transferred, and the column address indicates a transfer address pointer (TAP) address serving as a position initiating a new serial cycle after completion of transfer. From the next serial cycle (see time T₆ of FIG. 6B) when the external signal DT has risen, transferred data is outputted with the TAP address being a leading address. It is necessary that the rise timing (time T₅) of the external signal DT be given for a time period from the rise (time t₄) of the serial clock signal SC to the next rise thereof (time t₆). Accordingly, there are a restrictions in the time intervals t₁ (=T₅ -T₄) and t₂ (=T₆ -T₅). Furthermore, since the cycle time of the serial clock signal SC is 30 to 40 nsec, restrictions on the time intervals t₁ and t₂ are severe in application. To relax this, a split transfer system has been proposed.

This system is briefly described in NIKKEI ELECTRONICS No. 431, p. 126, Oct. 5, 1987.

This split transfer system will be described with reference to FIGS. 7A and 7B. The serial access memory 2 of the SAM port in which the split transfer system is used is split or divided into two sections of SAM (L) and SAM (U) as shown in FIG. 7A. These split SAM (L) and SAM (U) correspond to "0" and "1" of the most significant bit (hereinafter referred to as MSB) of the TAP address, respectively. Thus, independent data transfer can be made, respectively. The situation where a transfer cycle takes place when SAM (L) is subjected to serial access to carry out transfer of the row R of the memory array 1 of the RAM port will now be considered. In the same manner as in the case of FIG. 6A, the row address indicates the row R. The MSB of the TAP address is disregarded and is set to the MSB ("1" in this case) on the side where no serial access is conducted.

In this case, the SAM (U) on the side of the set MSB carries out the transfer operation. The transferred data is accessed from a TAP address where the MSB is newly set when serial access advances to shift from SAM (L) to SAM (U). In the case shown in FIGS. 7A and 7B, when a transfer cycle takes place for a time period of the serial addresses 0 to 127, the serial addresses 128 to 255 are subjected to transferring operation from the portion corresponding to the row R. When the serial access advances to 127 to enter the next SC cycle, the TAP address is accessed. Serial access is therefore continued further. As stated above, the split serial access memory section where serial access is carried out and the split serial access memory section where transfer is carried out are different from each other, and an address of the preceding cycle where an access to the TAP address is made is predetermined. Accordingly, restriction on the timings as in the case of FIGS. 6A and 6B does not exist.

Consideration will now be made of the method of constituting a data buffer for implementing high speed processing to display picture data using such a split transfer system.

As a random access memory of the DPRAM, a DRAM is used. For this reason, by using the page mode of DRAM, it is possible to access data of the same row in a time one half to one third of the time in the case where the row address changes. Further, the data of one row corresponds to data serially outputted from the SAM section, and the serial data serves as pixel data of a picture displayed. Therefore, how these pixels are arranged as a picture is important for high speed picture processing. From a viewpoint of processing of picture, employment a the method capable of processing, at a high speed, pixels in areas close to a square as much as possible permits high speed processing for any image pattern processing. Namely, high speed picture processing can be made in any direction of the longitudinal, lateral, or oblique directions. Thus, how data of one row which can be accessed in the page mode are allocated in a longitudinal direction with respect to the scanning direction of a picture displayed becomes important.

The case where a picture is composed of an array including four DPRAMs in a scanning direction and four DPRAMs in a longitudinal direction, i.e., an array of 4×4 is employed as a tile, will now be considered.

FIGS. 8A and 8B show the case where there is employed a DPRAM of the split transfer system having data of one row of 256 bits and the depth of a serial access memory (SAM) of 256 bits. Further, the picture size is assumed to be 1536 pixels in a scanning direction for brevity of explanation. In FIG. 8A, R0, R1, R2, . . . represent rows of a random access memory, respectively. L of 0 to 127 and U of 128 to 255 in the column direction represent columns where data are transferred to bisected SAMs in the split transfer, respectively. Since the section in the scanning direction is composed of four devices M₁, M₂, M₃, and M₄, 1536 pixels in the scanning direction correspond to data corresponding to three split segmented SAMs (=1536/(4×128)). Accordingly, when L and U of the segmented SAMs are allocated to pixels in a longitudinal direction, it is possible to conduct picture processing of an area closer to a square by an access in a page mode of one row. Namely, data of the RAM section are subjected to a split transfer in the order of R0L, R1U, R2L, R0U, R1L, R2U, . . . Since data are serially outputted from the SAM section to scan pixels, the configuration of data as a picture is such that the area indicated by slanting lines of FIG. 8B is composed of data of the row R0. This area can be subjected to an access of any portion in the page mode. Accordingly, the picture is covered with tiles of slanting lines shown in FIG. 8B such that it can be accessed in the page mode. Thus, high speed picture processing can be conducted.

Further, more detailed consideration of the data configuration in the scanning direction will now be made. How pixels are constructed from serial data of four DPRAMs is shown in FIGS. 9A and 9B. Data from the SAM 2 of the devices M1 to M4 are subjected to parallel-to-serial conversion as shown in FIG. 9A, and are outputted as serial data to constitute pixels one by one. Thus, it is sufficient to make a serial access from SAM 2 at a speed one fourth that of the serial out for the display of picture, thus making it possible to lessen the load on the SAM 2. Pixel data when such a parallel-to-serial conversion is carried out, are repeated in sequence as data from the DPRAM of M1, M2, M3, and M4 in order (see FIG. 9B). The frame of R0L, etc. of the type which has been described with reference to FIG. 8B is of such a data structure.

Let us now consider the case where the picture size is different from that in the above-described case. The case where the number of pixels in a scanning direction is 1024 is shown in FIGS. 10A and 10B. When it is assumed that the size of a tile used is 4×4 in the same manner as that in the above-described case, 1024 pixels correspond to the two segmented SAM sections (=1024/(4×128)).

At this time, for L and U of split data of one row are caused to correspond to the pixel arrangement in a longitudinal direction, it is necessary to carry out a split transfer in order as shown in FIG. 10A. Namely, when transfer is carried out in the order of R0L, R1U, R0U, R1L, . . . , the area indicated by slanting lines in FIG. 10B can be desirably accessed in the page mode. However, in the case of transfer at this time, L and U cross each other with respect to SAM. Accordingly, such a transfer cannot be carried out by a conventional DPRAM. Naturally, when the size of the tile is changed, a buffer can be constructed even for this picture size without cross transfer. However, the design of the buffer memory system must be changed for every picture size displayed. Further, since a cross transfer occurs unless the number of tiles in a scanning direction is necessarily odd as seen from the case of FIG. 8B, it may not be possible to select an optimum tile size of a memory capacity which is not wasteful with respect to a picture size.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide an image memory which has no necessity to vary a buffer memory system for every size of picture displayed.

According to the present invention, there is provided an image memory which includes a random access memory array capable of being randomly accessed; a serial access memory array partitioned into 2^(n) (n>1) divisional areas cyclically and serially accessed asynchronously with the random access memory. A data transfer unit transfers data between the random access memory array and the serial access memory array. A determining unit determines a row of data to be transferred from the random access memory array to each of the divisional areas. A designating unit designates at least one of a top serial access address and a last serial access address respectively of each divisional area. The data transfer unit executes data transfer from the random access memory array to the serial access memory array in accordance with outputs from the determining unit and the designating unit.

According to the image memory of this invention, the determining unit determines the order of serial accesses to divisional areas of a serial access memory array and a row of data to be transferred from a random access memory array to each of the divisional areas in such a manner that tiles of a display screen are constituted by one row of the random access memory array. The designating unit designates at least one of a top serial access address and a last serial access address respectively of each divisional area. The data transfer unit transfers data between the random access memory array and the serial access memory array. With this arrangement of the image memory, it is possible to deal with various sizes of display screens without changing the buffer memory system.

Furthermore, in the image memory constructed according to the invention, the random access memory is partitioned into memory blocks. The determining unit determines the order of serial accesses to divisional areas partitioned into an n power of 2 (n>1) of the serial access memory array and a row of data to be transferred from the random access memory array of each memory block in such a manner that tiles of a display screen are constituted by the data of the same rows of the memory blocks. The designating unit automatically designates at least one of a top serial access address and a last serial access address respectively of each divisional area. The data transfer unit transfers data between the random access memory array and the serial access memory array. With this arrangement of the image memory, it is possible to deal with various sizes of display screens without changing the buffer memory system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIGS. 1A and 1B are schematic diagrams showing an embodiment of an image memory according to this invention;

FIGS. 2 and 3 are schematic diagrams used for explaining the relationship between transfer data and TAP addresses;

FIGS. 4A and 4B are schematic diagrams used for explaining the method of processing tiles of the image memory according to the present invention, for the case where a display screen size cannot be filled in with tiles without leaving any space;

FIGS. 5A and 5B are schematic diagrams used for explaining the method of making the size of a tile fitted to a display screen size., by using the image memory of this invention;

FIGS. 6A and 6B are explanatory views for describing a conventional image memory,

FIGS. 7A and 7B are explanatory views for describing a conventional split transfer system;

FIGS. 8A and 8B are explanatory views for describing a conventional transfer method in which a pixel area close to a square as much as possible is accessible in a page mode cycle by the split transfer system;

FIGS. 9A and 9B are explanatory views for describing the correspondence between the SAM port and pixels in a scanning direction of four devices; and

FIGS. 10A and 10B are explanatory views for describing the problem of a conventional image memory in the case where there is employed a conventional transfer method in which a pixel area close to a square as much as possible is accessible in a page mode cycle by the split transfer system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B show an embodiment of an image memory according to the present invention. The image memory of this embodiment is constructed of 16 random access memory array devices 1i (i=0, . . . , 15), 16 serial access memory array devices 2i (i=0, . . . , 15), a data transfer unit 3, a determining unit 4, and a designating unit 5 (see FIG. 1A). The image memory stores display data to be displayed on a display screen 10 having 1024 pixels in the scan direction. Each SAM 2i is serially and cyclically accessed asynchronously with RAM 1i. SAM 2i is partitioned into four divisional areas in this embodiment. The determining unit 4 determines the order of serial accesses to the divisional areas of SAMs 2i and the row of the data to be transferred from RAM 1i to the divisional areas so as to constitute one tile of the display screen 10 by one row of data of 16 RAMs 1₁, . . . , 1₁₆. The designating unit 5 designates the start or last serial access address of each divisional area of SAM 2i. The data transfer unit 3 transfers data between RAM 1i and SAM 2i in accordance with the outputs from the determining unit 4 and designating unit 5.

The operation of the above-described embodiment will be described below. The four divisional areas of SAM 2i in each row of RAM 1i are represented by four columns P0, P1, P2, and P3 each composed of 64 bits. The number of pixels 1024 in the scan direction of the display screen 10 therefore corresponds to the number of bits of four (=1024/(4×64)) divisional areas of SAM 2i. The determining unit 4 determines the order of data transfer from RAM 1i to SAM 2i, for example, as R0P0, R1P1, R2P2, R3P3, R0P1, R1P2, R2P3, R3P0, R0P2, R1P3, R2P0, R3P1, R0P3, R1P0, R2P1, and R3P2. R0, R1, R2, and R3 represent the rows of RAM 1_(i). Thus, R0P0 for example represents the column P0 of the row R0 of RAM 1_(i). The designating unit 5 designates the start or last serial access address of each divisional area of SAM 2i. However, in this example, the screen can be filled with tiles without leaving any space as described later, so that an output from the designating unit 5 can be neglected. As the order of data transfer from RAM 1i to SAM 2i is determined by the determining unit as described above, the data transfer unit 3 transfers data in accordance with this determined order. The data transfer from SAM 2i to the display screen is then carried out in the following manner. First, four R0P0 data transferred from RAM 1i (i=0, . . . , 3) to SAM 2i (i=0, . . . , 3) is serial-parallel converted and transferred to the display screen 10 as illustrated in FIGS. 9A and 9B. Thereafter, four RJPK data (J, K=0, . . . , 3) transferred from RAM 1i (i=4, . . . , 7) to SAM 2i (i=4, . . . , 7) is serial-parallel converted and transferred to the display screen 10, and this operation is repeated as many times as necessary. Similarly, in accordance with the data transfer order determined by the determining unit, data transferred from RAM 1i (i=4, . . . , 7) to SAM 2i (i=4, . . . , 7) is transferred to the display screen 10. Similar operation is repeated until data transferred from RAM 1i (i=12, . . . , 15) to SAM 2i (i=12, . . . , 15) has been transferred to the display screen 10. In the above manner, a tile (an area indicated by hatched lines in FIG. 1B) is constituted by the data corresponding to one row of 16 RAMs 1i (i=0, . . . , 15). Accordingly, this tile can be accessed at a high speed in a page mode. In the above embodiment, the size of the display screen 10 has been assumed to be 1024 pixels in the scan direction. Whatever the size of the display screen is, the pixels in the scan direction can be constituted by a suitable combination of 64 bit divisional areas of SAM 2i. Furthermore, since data transfer can be controlled independently from each divisional area, it is not necessary to execute the cross split transfer.

Next, the situation where the screen cannot be filled in with tiles without leaving any space where it becomes necessary to precisely control TAP addresses designated by the designating unit 5 for the data transfer to each divisional area, will be described.

FIGS. 2 and 3 show examples of the relationship between the transfer data and TAP addresses. FIG. 2 illustrates the case where when an access to SAM 2i advances to a new divisional area, the top serial address of the new divisional area is indicated by a TAP address. While SAM (P0) having data already transferred from a row R0 and having serial addresses 0 to 63 is being accessed, if an instruction to transfer data from a row R2 is entered while designating a TAP 0 of SAM (P2), then data is transferred from the row R2 to SAM (P2) and after reading data up to the serial address 63, the serial address jumps to TAP 0. Next, while SAM (P2) having serial addresses TAP 0 to 191 is being accessed, if an instruction to transfer data from a row R3 is entered while designating a TAP 2 of SAM (P1), then data is transferred from the row R3 to SAM (P1) and after reading data up to the serial address 191, the serial address jumps to TAP 2. Next, while SAM (P1) having serial addresses TAP 2 to 127 is being accessed, if an instruction to transfer data from the row R1 is entered while designating a TAP 1 of SAM (P3), then data is transferred from the row R1 to SAM (P3) and after reading data up to the serial address 127, the serial address jumps to TAP 1. While SAM (P3) is being accessed, if there is no transfer instruction, after reading data up to the serial address 255, the serial address circulates back to the serial address 0.

FIG. 3 illustrates the case where when an access to SAM 2i advances to a new divisional area, the last serial address of the new divisional area is indicated by a TAP address. While SAM (P0) having data already transferred from a row R0 and having serial addresses 0 to 63 is being accessed, if an instruction to transfer data from a row R2 is entered while designating a TAP 0 of SAM (P2), then data is transferred from the row R2 to SAM (P2) and after reading data up to the serial address 63, the serial address jumps to an address 128. Next, while SAM (P2) having serial addresses 128 to TAP 0 is being accessed, if an instruction to transfer data from a row R3 is entered while designating a TAP 2 of SAM (P1), then data is transferred from the row R3 to SAM (P1) and after reading data up to the serial address TAP 0, the serial address jumps to an address 63. Next, while SAM (P1) having serial addresses 64 to TAP 2 is being accessed, if an instruction to transfer data from the row R1 is entered while designating a TAP 1 of SAM (P3), then data is transferred from the row R1 to SAM (P3) and after reading data up to the serial address TAP 2, the serial address jumps to an address 192. After the serial address TAP 1, the access circulates back to the serial address 0.

In the above description, either a top address or a last address is designated by a TAP address. It is understood that both addresses may be selectively designated at each data transfer. With the above TAP address control, it becomes possible to accommodate with a display screen having an optional size. In the case where the display screen cannot be filled in with tiles without leaving any space in the scan direction as shown in FIGS. 4A and 4B, it becomes possible to make the number of pixels coincide with the number of serial accesses, by selecting suitable top TAP addresses for the last tile as shown in CASE 1 (refer to FIG. 4A) or by selecting suitable last TAP addresses for the last tile as shown in CASE 2 (refer to FIG. 4B). Alternatively, the size of tiles may be changed to fill tiles in a display screen without leaving any space, as shown in FIGS. 5A and 5B. It becomes possible to realize divisional areas of SAM 2i having a number of serial accesses coincident with the number of pixels of changed tiles, by selecting suitable top TAP addresses for the tiles as shown in CASE 1 (refer to FIG. 5A) or by selecting suitable last TAP addresses for the tiles as shown in CASE 2 (refer to FIG. 5B). The number of tiles in the scan direction in both the cases may be even or odd without any restriction, so that it is possible to use a memory capacity most suitable for a display screen size.

Lastly, the case where a TAP address designated during the transfer cycle is at the divisional area of SAM 2i which is now being serially accessed will be considered. In this case, the divisional area of SAM 2i under serial access is the same divisional area of SAM 2i to which data transfer is carried out. In order to avoid such conditions, one of the following three approaches is made.

(1) Timings of data transfer are synchronized, as conventional, to execute the transfer from RAM 1i to SAM 2i and the serial output from SAM 2i.

(2) Transfer from RAM 1i to SAM 2i is neglected.

(3) As to the bits of a TAP address, the contents of bits designating a divisional area of SAM 2i are neglected, and the bits for the next divisional area to that of SAM 2i under serial access are set.

The next divisional area herein means a divisional area to be accessed next when there is no transfer cycle and the serial accesses are sequentially executed.

In the above description, SAM 2i has been partitioned into four divisional areas. It is understood that the number of divisions may take other n powers 2 (n>1). As the number of divisions increases, the number of transfer gates will increase. The optimum number is therefore 4, 8, 16 and the like.

Furthermore, in the foregoing description, data at a row of RAM 1i is divisionally transferred to a divisional area of SAM 2i. A divisional RAM 1i may be provided in correspondence with each divisional area of SAM 2i. In such a case, although each divisional RAM 1i is designated by the same row address, data from each divisional RAM 1i is controlled to be selectively transferred. In this case, the bit structure of RAM 1i and SAM 2i becomes different (e.g., ×16 for RAM 1i, and ×4 for SAM 2i). 

What is claimed is:
 1. A video memory comprising:a first random access memory array A1 having first to fourth rows; a second random access memory array A2 having fifth to eighth rows; a third random access memory array A3 having ninth to twelfth rows; a fourth random access memory array A4 having thirteenth to sixteenth rows; each of said first to fourth random access memory arrays having a plurality of memory cells arranged in matrix form, cells at a same row of an array being accessed simultaneously; a first serial access memory array B1; a second serial access memory array B2; a third serial access memory array B3; a fourth serial access memory array B4; said first to fourth serial access memory arrays being provided corresponding to said first to fourth random access memory arrays, respectively, and temporally storing data corresponding to cells at one row of said random access memory arrays; means for transferring data in a row from A1 to B1, from A2 to B2, from A3 to B3, and from A4 to B4; and means for sequentially reading data from B1, B2, B3 and B4.
 2. A video memory according to claim 1, wherein said means for sequentially reading has a designating unit to designate a top address of serial access for said first to fourth serial access memory arrays.
 3. A video memory according to claim 1, wherein said means for sequentially reading has a designating unit to designate a last address of serial access for said first to fourth serial access memory arrays.
 4. A video memory according to claim 1, wherein said means for sequentially reading has a designating unit to designate a top address of serial access and a last address of serial access for said first to fourth serial access memory arrays.
 5. A video memory according to claim 1, wherein said means for sequentially reading reads data in an order of B1, B2, B3, B4, B2, B3, B4, B1, B3, B4, B1, B2, B4, B1, B2 and B3.
 6. A method of reading out data from a video memory havinga first random access memory array A1 having first to fourth rows, a second random access memory array A2 having fifth to eighth rows, a third random access memory array A3 having ninth to twelfth rows, and a fourth random access memory array A4 having thirteenth to sixteenth rows, each of said first to fourth random access memory arrays having a plurality of memory cells arranged in matrix form, cells at a same row of an array being accessed simultaneously; and a first serial access memory array B1, a second serial access memory array B2, a third serial access memory array B3, and a fourth serial access memory array B4, said first to fourth serial access memory arrays being provided corresponding to said first to fourth random access memory arrays, respectively, and temporally storing data corresponding to cells at one row of said random access memory arrays, said method comprising the steps of:transferring data from the first row of A1 to B1; transferring data from the second row of A2 to B2 while reading data from B1; transferring data from the third row of A3 to B3 while reading data from B2; transferring data from the fourth row of A4 to B4 while reading data from B3; transferring data from the first row of A2 to B2 while reading data from B4; transferring data from the second row of A3 to B3 while reading data from B2; transferring data from the third row of A4 to B4 while reading data from B3; transferring data from the fourth row of A1 to B1 while reading data from B4; transferring data from the first row of A3 to B3 while reading data from B1; transferring data from the second row of A4 to B4 while reading data from B3; transferring data from the third row of A1 to B1 while reading data from B4; transferring data from the fourth row of A2 to B2 while reading data from B1; transferring data from the first row of A4 to B4 while reading data from B2; transferring data from the second row of A1 to B1 while reading data from B4; transferring data from the third row of A2 to B2 while reading data from B1; transferring data from the fourth row of A3 to B3 while reading data from B2; and reading data from B3.
 7. A method according to claim 6, wherein a top address of each serial access memory is designated at each reading.
 8. A method according to claim 6, wherein a last address of each serial access memory is designated at each reading.
 9. A method according to claim 6, wherein top and last addresses of each serial access memory are designated at each reading.
 10. A video memory comprising:a random access memory array partitioned into N (N=2^(n), n is an integer greater than or equal to 2; N is an integer of 4 or greater) divisional RAM areas, each of said divisional RAM areas including a plurality of memory cells arranged in a matrix, cells at a same row of the matrix in a divisional RAM being accessed simultaneously; a serial access memory array partitioned into N divisional SAM areas serially accessed asynchronously with said random access memory array, each of said divisional SAM areas corresponding to one of said divisional RAM areas and temporarily storing data from cells of one row of said matrix of the divisional RAM areas; and a data transfer unit to transfer data between a divisional RAM area of said random access memory array and its corresponding divisional SAM area of said serial access memory array; wherein said serial access is executed by reading one divisional SAM area at one time, and when present reading reaches an end point, a next reading is performed in another divisional SAM area, from an address which is designated during said present reading.
 11. A video memory according to claim 10, wherein if at least one of a top serial access address and a last serial access address is at a divisional SAM area of said serial access memory array which is to be serially accessed, then a serial output from said serial access memory array is executed after data transfer from said random access memory array to said serial access memory array has been completed.
 12. A video memory according to claim 10, wherein if at least one of a top serial access address and a last serial access address is at a divisional SAM area of said serial access memory array which is to be serially accessed, then a divisional SAM area of said serial access memory array to be sequentially accessed next is designated for access.
 13. A video memory comprising:a random access memory array, partitioned into N (N=2^(n), n is greater than or equal to 2; N is an integer of 4 or greater) divisional RAM areas, each of said divisional RAM areas including a plurality of memory cells arranged in a matrix, cells at a same row of the matrix in a divisional RAM being accessed simultaneously; a serial access memory array, partitioned into N divisional SAM areas serially accessed asynchronously with said random access memory array, each of said divisional SAM areas corresponding to one of said divisional RAM areas and temporarily storing data from cells of one row of said matrix of the divisional RAM areas; and a data transfer unit to transfer data from a divisional RAM area of said random access memory array to its corresponding divisional SAM area of said serial access memory array; wherein serial access is executed by reading one divisional SAM area at one time, and when present reading reaches an end point, a next reading is performed in another divisional SAM area, from an address which is designated during said present reading.
 14. A video memory according to claim 13, wherein said divisional RAM areas of said random access memory array are independently addressed.
 15. A video memory according to claim 13, wherein said divisional RAM areas of said random access memory array are commonly addressed.
 16. A video memory comprising:a random access memory array, partitioned into N (N=2^(n), n is greater than or equal to 2; N is an integer of 4 or greater) divisional RAM areas, each of said divisional RAM areas including a plurality of memory cells arranged in a matrix; a serial access memory array, partitioned into N divisional SAM areas continuously accessed asynchronously with respect to said random access memory array, each of said divisional SAM areas temporarily storing data from cells of one row of said matrix of the divisional RAM areas; and a data transfer unit to transfer data from a divisional RAM area of said random access memory array to its corresponding divisional SAM area of said serial access memory array; wherein serial access of a divisional SAM area jumps to another divisional SAM area to start a new serial access when present serial access reaches an address which is a boundary.
 17. A video memory comprising:a random access memory array, partitioned into N1 (N1=2^(n), n is an integer; N1 is an integer of 2 or greater) divisional RAM areas, each of said divisional RAM areas comprising a plurality of memory cells arranged in a matrix, cells at a same row being accessed simultaneously; a serial access memory array, partitioned into N2 (N2=2^(n), n is greater than or equal to 2; N2 is an integer of 4 or greater) divisional SAM areas continuously accessed asynchronously with respect to said random access memory array, at least some of said divisional SAM areas corresponding to at least some of said divisional RAM areas and temporarily storing data corresponding to cells at one row of the matrix of the divisional RAM areas; and a data transfer unit to transfer data from a divisional RAM area of said random access memory array to said serial access memory array; wherein serial access of a divisional SAM area jumps to another divisional SAM area to start a new serial access when present serial access reaches an address which is a boundary. 